Catheter ablation is a therapy for various cardiac arrhythmias. Cardiac arrhythmia is a condition in which an organism's heart beats too quickly, too slowly, or in an irregular (e.g., non-periodic) manner. A more precise name for the condition in which the heart beats too quickly is “tachycardia.” A diagnosis of tachycardia may be given for a heartrate above a certain value (e.g., 100 beats per minute, or bpm, in adults). Tachycardia that originates in the ventricular region of the heart may be referred to as “ventricular tachycardia,” or “VT.”
One variety of VT is named “reentrant VT.” Reentrant VT is characterized by improper electrical-signal propagation through the heart-muscle cells, or “cardiomyocytes.” This may occur when cardiomyocytes are stressed or irritated by, for example, ischemia (i.e., an inadequate blood supply caused by, for example, a myocardial infarction), tissue necrosis, drug reactions, or electrolyte imbalances. Such irritation may alter conduction patterns in the heart and/or the conduction speed and refractory period of the affected cardiomyocytes, resulting in improper electrical-signal propagation. The improper pathways the electrical signal takes are “reentrant pathways” or “reentrant circuits.” Reentrant pathways may be created also when a scar develops on the heart tissue, such as following a myocardial infarction. Reentrant pathways are closed pathways (e.g., a loop or circuit).
When the electrical signal takes a reentrant pathway, the ventricles may contract and cause a heartbeat at an improper time and rate. A second propagation of an electrical signal over heart tissue following a recent first propagation does not, however, necessarily trigger an extra heartbeat; no contraction of the ventricles is triggered if an electrical signal propagates during a refractory period of the cardiomyocytes. A scar on the heart tissue, however, may create a large obstacle for the electrical signal to propagate around. If the path around the scar is long enough, the electrical signal may take longer to travel around the scar than the refractory period of the cardiomyocytes. This may result in the cardiomyocytes around the scar being constantly activated by the electrical signal travelling on a reentrant pathway around the scar, thus resulting in an arrhythmia. A scar on the heart tissue may also itself conduct electrical signals through microchannels therein, potentially forming reentrant pathways.
Catheter ablation is the process of using a catheter to, among other things, burn or freeze cardiomyocytes that form a reentrant pathway, creating scar tissue that will not conduct or causing a scar to conduct much less than before ablation. Severing the reentrant pathway may prevent the electrical signal from propagating down the reentrant pathway and triggering a heartbeat at an improper time and rate.
Determining the location or locations for ablation on the heart tissue presents various difficulties. In making this determination, one may seek to minimize the amount of heart tissue burned or frozen when attempting to treat the tachycardia. Each lesion intentionally created by burning or freezing involves a risk of collateral injury to the heart, such as, for example, one or more steam pops, perforations, and tamponades. Instead or in addition, one may seek to minimize the number of ablations in order to limit the duration of the ablation procedure. Longer ablation procedures are associated with increased risk of chamber perforation, thromboemboli, bleeding, and radiation overexposure.
Because reentrant pathways are three-dimensional (3D) and may assume complicated shapes, determining the location or locations of ablation such that their number and size is minimized presents difficulties. One method is to insert a catheter into the heart and record the voltage at the catheter's tip at each position in space to build a static surface representation of the heart and the voltages thereon. The resulting images can be difficult to understand because the electrical signal changes constantly on the heart's surface and such changes may not be captured by the static surface representation. Additionally, there is no consensus in the medical community on the proper method for picking ablation locations based on the static surface representations. This method and other methods requiring catheter-based mapping are very time-consuming and error-prone because the catheter must be placed on or near every relevant spot on the heart. In addition, these methods do not ignore pathways that are not reentrant (e.g., dead-end pathways). Another method for determining the location or locations of ablations comprises stimulating cardiac tissue with a catheter tip and recording an electrocardiograph (ECG). The recorded ECG may be compared to a second ECG that captures the arrhythmic activity to attempt determining whether the catheter tip was on a reentrant pathway. Even if this comparison reveals a reentrant pathway, it does not indicate where to ablate.
Other methods using software-run 3D simulations are being explored to determine ablation targets. Using software-run 3D simulations may overcome many of the deficiencies of the foregoing methods. Software-run 3D simulations may overcome many of the deficiencies because they enable processing of large amounts of information in short periods of time without manual human intervention, leading to fewer missed spots and errors in data collection inherent in the previously discussed methods. Using simulations, unlike other methods, may not require catheter mapping or heart stimulation. Currently, however, when presented with simulation data describing the electrical-signal propagation on and through heart tissue over time in a 3D simulation, clinicians may have trouble picking ablation locations such that the number and size of ablations is minimized. To pick an ablation location, electrical-signal levels may need to be repeatedly observed in the simulation at many or all locations in the ventricle simulation at many or all points in time for which data is available. Picking an ablation location with this process is further complicated by the fact that the electrical signal may propagate not only along the inner and outer surface of the tissue but also within or through the tissue. Picking an ablation location by observing the simulation may also be difficult because the simulation shows electrical-signal propagation along pathways that eventually get blocked by, for example, a scar or cardiomyocytes in their refractory period. Such blocked propagations may obscure actual reentrant pathways the clinicians seek to find in the simulation. FIG. 1A and FIG. 1B are illustrations of exemplary 3D simulation views of electrical-signal propagation on and through a heart 100a and 100b, respectively. A clinician may attempt to observe views 100a and 100b to determine optimal ablation locations. Catheter-ablation locations (i.e., ablation targets) are optimal if they are at locations requiring the fewest and smallest ablations to treat and prevent future arrhythmia. Views 100a and 100b show electrical-signal propagations that may eventually get blocked by, for example, scar tissue, and therefore highlight pathways that are not reentrant pathways. Additionally, a clinician may need to observe cross sections of views 100a and 100b to study the electrical signal propagation through the heart tissue's interior. Because of the quantity of information presented in views 100a and 100b, isolating the reentrant pathways and finding the optimal ablation locations may be very time-consuming or impossible.
The requisite observations in a software-based simulation may be very time-consuming and, if many locations are not observed at many points of time, may lead to over-selection of locations for ablation than necessary to treat the VT, or selection of locations that do not help treat the VT. Even if a clinician observes the electrical-signal propagation at all locations at all points of time, he or she may need to consider this enormous amount of information simultaneously to determine the proper locations to ablate and the proper ablation size. Success in this process requires much training, intuition, and expertise on the part of the clinician. This decreases the number of clinicians available to perform the analysis and increases procedure costs. Even with requisite training and expertise, clinicians may make errors when picking ablation locations based on observations of a simulation. The use of software-based simulations may have the benefit of providing clinicians with more data and data that is more accurate, but it may overwhelm and obscure the information the clinician is looking for.
The disclosed systems and methods are directed to overcoming one or more of the problems set forth above and/or other problems or shortcomings in the prior art.